1. Field of the Invention
The present invention relates to a steerable front wheel suspension for a vehicle and, more particularly, to a steerable front wheel suspension for a vehicle which can enhance riding comfort and handling safety by providing a nominal kingpin, simultaneously with providing long upper control arms.
2. Description of Related Art
A suspension for vehicle connects an axle shaft and a car body to each other, and controls a position of a tire with respect to the car body for its optimal position during running of the vehicle. Thus, optimal handling safety may be obtained, and the axle shaft is prevented from directly transmitting impact or vibration from a road surface to the car body. The suspension also prevents damage of baggage, thereby improving riding comfort.
Particularly, in a steerable front wheel suspension, the suspension should be designed to absorb vibration or impact from the road surface as well as obtain running safety by maintaining an optimal position of a vehicle's straight ahead position and handling safety of a vehicle's turning position.
The suspension system is structurally characterized as an integral shaft suspension and an independent suspension. The suspension of the present invention concerns an independent suspension, and the independent suspension includes several kinds of suspension systems. There are McPherson strut type and Wishbone type suspension systems used in the front wheels.
The front wheel is mounted to change its direction from right to left or from left to right, centering on the kingpin, and is also mounted with a geometric angle so as to meet the requisite for the front wheel suspension.
Mounting the front wheel with the geometric angle is called a front wheel alignment. The front wheel alignment is distinguished by several elements, but the optimal operation of the front wheel is, however, achieved by the supplemental operation of the elements with respect to each other. A kingpin inclination among the elements reduces a handling force of a steering wheel with a camber, absorbs an impact generated when the vehicle is running and being braked, and then increases a restitution force of a steering wheel, thereby obtaining running safety in a vehicle's straight ahead position and handling safety in a vehicle's turning position.
The kingpin is mounted at an inclination such that its central line lies at a predetermined angle with respect to the vertical line when the vehicle is viewed from the front. An interval between the central line of the kingpin and a center line of the wheel is called offset. The offset is generally characterized as an offset at the wheel center and an offset at the ground. The offset at the wheel center has an effect on the vehicle's straight ahead characteristics when either the driving force or the engine brake is applied to the wheels. The offset at the ground adversely affects the handling safety when the vehicle is braked and turning. If the offset at the ground is decreased, the sensitivity with respect to the steering is decreased, such that the handling safety is increased when the vehicle is cornering and being braked.
Further, steering stability of the vehicle is closely related to the vibration of the car body. During the vehicle's running, rolling, pitching, and yawing occurs in addition to bounce. These vibrations should be absorbed to increase the stability and the ride comfort.
In such vibrations, the rolling occurs at a predetermined point when the vehicle is in a turning situation, and this point is called a roll center. The variation rate of the roll center's height has an effect on the running safety and steering stability of the vehicle. Accordingly, to achieve running safety, it is more desirable to decrease the variation rate of the roll center's height.
FIG. 6 is a front view of conventional Wishbone type (strut assembly type) suspension, wherein the suspension includes upper and lower control arms 2, 4; a steering knuckle 6; a strut assembly 8 having a shock absorber 81 and a spring; and ball joints 21, 41 of the upper and lower control arms 2, 4. A kingpin offset a may be reduced by moving a car body-side connecting portion 22 of the upper control arm 2 to the engine room and moving the ball joints 41 of the lower control arm 4 to the outside, even though this suspension has an advantage in its simple structure and weight. To reduce the kingpin offset .alpha. or attain a negative (-) state, when the connecting point 22 of the upper control arm 2 is moved to the engine room, the available volume of the engine room is to be reduced simultaneously with increasing the kingpin offset .alpha.. The cornering performance of the vehicle may be adversely affected thereby. And also, it is impossible to move the ball joint 41 of the lower control arm 4 outwardly because of interference with a disk brake. Therefore, this suspension has problems in reducing the kingpin offset .alpha..
Further, if the length of the upper control arm is shortened, the kingpin offset may be almost zero, with an increased bushing effect and change in the camber angle. On the contrary, if the length of the upper control arm becomes long, the bushing effect can be decreased but the change in the camber angle and kingpin offset cannot he reduced to zero.
That is, the conventional suspension system cannot solve the combined problems of the kingpin offset, steering angle, and bushing effect at the same time.